Thermoregulatory sweating is well known to depend upon core body temperature (e.g. Benzinger, T. H. 1959 On physical heat regulation and the sense of temperature in Man. PNAS 45, 645-659). However, its precise control during exercise has proved difficult to quantify (see Kondo N., Nishiyasu, T., Inoue, Y., Koga, S. 2010. Non-thermal modification of heat-loss responses during exercise in humans. Eur J Appl Physiol 110, 447-458 for a recent review). Early work (Saltin, B. and Hermansen, L. 1966. Esophageal, rectal, and muscle temperature during exercise. J Appl Physiol 21(6) 1757-1762) showed that body temperature rose in proportion to an individual's capacity to do work, whilst sweating rose in proportion to the absolute work rate. However, Saltin & Hermansen (1966) provided no means by which these measurements might be combined to reveal {dot over (V)}O2 max.
Subsequent researchers investigated these two relationships in more detail demonstrating the importance of {dot over (V)}O2 max in determining the relationship between body temperature and sweating. One publication (Greenhaff, P. L. 1989. Cardiovascular fitness and thermoregulation during prolonged exercise in man. Br J Sports Med 23(2) 109-114) suggested 55% of the sweat loss could be accounted for by {dot over (V)}O2 max and another publication (Havenith, G. and van Middendorp, H. 1990. The relative influence of physical fitness, acclimatization state, anthropometric measures and gender on individual reactions to heat stress. Eur J Appl Physiol Occup Physiol 61(5-6) 419-427) showed the importance of many other parameters including body fat, surface area and acclimation.
These complexities result in various computational models of thermoregulation that do not allow for a simple derivation of {dot over (V)}O2 max from physiological parameters (see Havenith, G., Luttikholt, V. G. and Vrijkotte, T. G. 1995. The relative influence of body characteristics on humid heat stress response. Eur J Appl Physiol Occup Physiol 70(3) 270-279; Fiala, D., Lomas, K. J. and Stohrer, M. 1999. A computer model of human thermoregulation for a wide range of environmental conditions: the passive system. J Appl Physiol 87(5) 1957-1972; and Zhang, H., Huizenga, C., Arens, E. and Yu, T. 2001. Considering individual physiological differences in a human thermal model. Journal of Th 26 401-408). Indeed, non-thermal modulation of sweating would appear to rule out a simple relationship between sweating and body temperature that was dependent upon {dot over (V)}O2 max alone (see Yamazaki, F. et al. 1996. Responses of sweating and body temperature to sinusoidal exercise in physically trained men. J. Appl. Physiol., 80(2), 491-495; Shibasaki, M. and Crandall, C. G. 2011. Mechanisms and controllers of eccrine sweating in humans. Front. Biosci. 2, 685-696).
Methods and apparatus for either estimating or calculating {dot over (V)}O2 max (defined as the maximal oxygen uptake per unit time) are known. {dot over (V)}O2 max may also be known as maximal oxygen consumption rate, peak oxygen uptake or aerobic capacity and is the maximum capacity of an individual's body to transport and use oxygen during exercise. The name is derived from {dot over (V)} which represents volume per time and O2 representing oxygen. The dot above the V denotes rate of ventilation. {dot over (V)}O2 max can be defined either as the maximal oxygen uptake per unit time (ml min−1) or it can be expressed per unit body mass (ml min−1 kg−1). The ml min−1 definition is useful when considering absolute power output of an individual whereas the ml min−1 kg−1 is usually used when considering an individual's potential athletic ability. It is possible to provide an estimate of an individuals {dot over (V)}O2 max without performing any exercise testing using anthropometric data (e.g. an individual's weight, age, height, sex and amount of body fat). However, these estimates have limited accuracy since they do not take into account the effects of training and other physiological differences (e.g. heart disease). It is possible to improve anthropometric estimates of {dot over (V)}O2 max using estimates of training (e.g. questionnaires concerning the intensity and duration of exercise) but, such techniques both rely on accurate knowledge of an individual's training and assume a standard training effect of the reported exercise intensity on {dot over (V)}O2 max.
The known methods calculate {dot over (V)}O2 max from spirometry and measurements of oxygen concentration during ramped increases in exercise intensity to exhaustion. There are two classes of indirect assessments of {dot over (V)}O2 max, either using spirometry and oxygen measurements at non-maximal intensities followed by extrapolation to a theoretical maximum or by performance on one of many exercise tests (e.g. Cooper {dot over (V)}O2 max test, Astrand treadmill test).
Whilst spirometry with oxygen measurement during maximal exertion is, by definition, the only true way that {dot over (V)}O2 max can be determined it presents several major problems. First, the equipment to measure oxygen concentrations is expensive and requires careful maintenance since it directly interferes with breathing during a period of intense stress on the ventilatory system. The facemasks and tubes used make the test unpleasant and can restrict performance. Second, maximal exertion is required. Whilst maximal efforts can be obtained from experienced athletes the test is not suitable for the general population or those with health disorders where such intense efforts are not well tolerated, or may even be undesirable.
Exercise performance assessments of {dot over (V)}O2 max are equally problematic since they too require maximal exertion and the estimate of {dot over (V)}O2 max will depend upon the modality of the exercise chosen and the efficiency of the individual at that particular modality (e.g. an efficient runner with a low {dot over (V)}O2 max may obtain a similar assessment as an inefficient runner with a {dot over (V)}O2 max). Thus, an exercise performance {dot over (V)}O2 max assessment is limited in its usefulness since it does not differentiate between poor circulatory performance or poor technique.
Despite the difficulties identified above, {dot over (V)}O2 max testing is a popular means to measure cardiovascular fitness. The present invention recognises the need for an improved technique for assessing {dot over (V)}O2 max which is suitable for all users.